Such a pulse-width modulation control may be a low-frequency control, which constitutes a relatively inexpensive approach up to 100 Hz. Additional costs arise if the pulse-width modulation frequency is shifted into an inaudible high-frequency ultrasonic range above 20 kHz, the more so since the EMC specifications (electromagnetic compatibility) to be observed in this case are difficult to satisfy, so that this method is currently not used much.
The conventional cost-effective control methods generate electromechanical sound emissions as a result of the related low-frequency sampling of the pulse-width modulation. The generated sound emissions have high energy and a narrow bandwidth. Because of the narrow band width and the high energy, these sound emissions are clearly audible to the human ear. In particular when the ambient noise is lower, the interfering narrow-band noise of the electric motor drowns out the ambient noise due to the pulse-width modulated control, so that it is virtually impossible to ignore this interfering noise.
Japanese patent document JP 2006191756 A discusses a control method of a DC motor, in which a spectral diffusion modulation of the PWM frequency is implemented for the purpose of noise suppression.